1. Field of the Invention
The present invention relates to an electric motor comprising a thermistor-type overcurrent protection device.
2. Description of the Prior Art
It is a known practice to protect the windings of an electric motor by connecting them in series with a thermistor having a positive temperature coefficient, that is to say an ohmic value which is an increasing function of temperature. When the current intensity increases in the circuit, the thermistor undergoes a temperature rise which tends to reduce the supply current as well as the terminal voltage of the excited winding possibly to such an extent that this winding is put out of service.
A device of this type is employed in particular in electronically switched direct-current motors having the function of driving fans for cooling electrical appliances. In this case the motor usually has two windings supplied alternately under the control of a rotor-position detector. In the known devices, use is made of a single thermistor in series with the two windings which are in turn in parallel with respect to each other. When the rotor is brought to a standstill by an obstacle, the thermistor heats up and causes a voltage drop within both windings at the same time. In the case of a cooling fan, the motor load is zero at zero speed and cooling of the thermistor permits re-starting of the motor when the disturbance has ceased. However, as long as the thermistor has not returned to its normal value, it continues to influence the supply voltage of the two windings. The result is that re-starting of the fan takes place slowly or even that, in extreme cases, the residual voltage is not sufficient to ensure correct re-starting of the motor.